Microspheres as bond line spacers in epoxies

Today’s electronics are demanding tighter and tighter tolerances, and the assembly of many items require holding precise spacing between parts during assembly.  The spherical shape and precise sizes available make microspheres the ideal candidate as a precision spacer in various liquid adhesives and epoxies.

Important considerations:

1) The actual bond line will correspond to the largest spheres, not the average size.  Narrow size ranges of a few micron ensure the proper gap is maintained.

• If a 30um gap is required use a spacer grade size range of 27-30 micron.
• If a 40um gap is required use a spacer grade size range of 37-40 micron.
• If a 53um gap is required use a spacer grade size range of 50-53 micron.

3) Functionality gained from using different materials.

• Glass offers the best mechanical and chemical stability at a wide range of temperatures.
• Plastic such as PMMA can be used where some deformation is desirable.
• Metal coated glass spheres can be used where conductivity is desirable.
• Hollow Glass spheres can be used where assembly pressures are low, and reduced thermal conductivity is desired, available as uncoated hollow glass or silver coated hollow glass spheres.

4) The importance of sphere loading (% spheres by volume in the adhesive)

• Theoretical maximum loading by volume for a monolayer is 61%
• A mixture of about 5% by volume should work for most applications.
• Narrow bond lines with high assembly forces will require higher loadings
• Low crush strength spheres will require higher loadings.
• Proper dispersion in the adhesive will help to minimize the loading needed.

5) Adhesive / Epoxy selection

• High viscosity epoxies will help maintain sphere dispersion.
• For best results choose an adhesive that adheres to the spheres and the base material.
• Long pot-life materials work best, as they allow excess adhesive to flow out of the bond line during assembly.

6) Spacer Availability – Cospheric LLC stocks a wide variety of sizes and materials, and can custom produce spacer grade microspheres for your application.

• Spacer Grade Solid Glass Spheres, narrow 3um ranges, >95% in range, 22-53 micron
• Silver Coated Solid Glass Spheres, >90% in range, 22-110 micron sizes
• Soda Lime Glass Spheres, >90% in range, 27 micron to 3.4 mm sizes
• Solid PMMA Spheres, >90% in range, 20-150um sizes

Applications for Bond Line Spacers

Spacer Grade Glass Microspheres are presently used in gas plasma displays, automotive mirrors, electronic displays, flip chip technology, filters, sporting goods equipment, calibration standards and transformer manufacturing.  Every day engineers are finding new and innovative uses for bond line spacers.  One area that has had the most publications is in die attachment in the semi-conductor industry, a particularly interesting area is in using spacers for building multi-die packages. Continue reading “Microspheres as bond line spacers in epoxies” »

BioCompatability of Metal Coated Spheres

For those scientists who are looking to use silver coated materials such as silver coated microspheres in biomedical applications, it is important to understand whether they are bio-compatable.  A selection of abstracts and article references related to the biocompatability of silver follow:

The Biocompatibility of Silver²

The experiments reported have referred to some of the characteristics of the biocompatibility of Ag. Silver has been shown to display interactions with albumin, as an example of a plasma protein, quite different from those of most metals. Such studies shed further light on the complex issue of protein adsorption on biomaterials. It has also been demonstrated that Ag at concentrations < 1 ppm exerts a considerable influence on the activity of lactate dehydrogenase, this effect being reversed in the presence of albumin. A significant but transient increase in blood levels of Ag following intramuscular implantation of the metal has been observed. This is not reflected in any raised urine level. It is proposed that the richly vascular tissue immediately surrounding the implant in the acute phase of the response gives rise to the transient increase, but a subsequent decrease in vascularity reduces this possibility. It appears that Ag released from implants following this initial period substantially remains in the local area.²

Lack of toxicologocial side-effects in silver-coated megaprostheses in humans¹

Deep infection of megaprostheses remains a serious complication in orthopedic tumor surgery. Furthermore, reinfection gets a raising problem in revision surgery of patients suffering from infections associated with primary endoprosthetic replacement of the knee and hip joint. These patients will need many revision surgeries and in some cases even an amputation is inevitable. Silver-coated medical devices proved their effectiveness on reducing infections, but toxic side-effects concerning some silver applications have been described as well. Our study reports about a silver-coated megaprosthesis for the first time and can exclude side-effects of silver-coated orthopedic implants in humans. The silver-levels in the blood did not exceed 56.4 parts per billion (ppb) and can be considered as non-toxic. Additionally we could exclude significant changes in liver and kidney functions measured by laboratory values. Histopathologic examination of the periprosthetic environment in two patients showed no signs of foreign body granulomas or chronic inflammation, despite distant effective silver concentrations up to 1626 ppb directly related to the prosthetic surface. In conclusion the silver-coated megaprosthesis allowed a release of silver without showing any local or systemic side-effects.¹

Specific Article References for the biocompatability of silver are below: See the References

Metal Coated Microspheres – Conductive Silver Coating

From early days engineers have been looking for ways to shield circuits from electromagnetic interference (EMI). One of the most effective methods of shielding is by creating an electrically conductive enclosure around the circuit or device. This can be accomplished by using any electrically conductive material. Advances in coated microspheres have enabled the creation of light weight electrically conductive coatings that provide excellent EMI shielding.

Electrically conductive microspheres are produced by applying a metallic silver coating to the surface of the microspheres, thus giving the advantages of a metal particle with the additional properties of the core microsphere.  Typically hollow glass microspheres are silver coated as this offers the combination of a low density filler and a conductive particle.   Coatings with EMI shielding of greater than 45db have been produced by adding as little as 20% by weight of M-18 silver coated microspheres.

Cospheric offers metal coated (silver)  electrically conductive microspheres in a variety of sizes and densities as shown in the table below, custom particle size ranges are also available: